Presently, NOx, SPM (Suspended Particulate Matter), and photochemical oxidant cause air pollution, and the influence on the environment is regarded as a problem. For example, the photochemical oxidant mainly containing a strongly acidic substance such as ozone is produced by a photochemical reaction caused by a pollutant such as NOx or hydrocarbon exhausted from factories, offices, and automobiles when the pollutant is irradiated with the sun light, and causes photochemical smog.
In Japan, environmental standards are set for these substances, and general environmental air measurement stations in various places measure the substances. For example, an environmental standard is set for the concentration of the photochemical oxidant, and general air environment observation stations in various places perform gas concentration measurements by an automatic measurement method such as an ultraviolet absorption method. Note that the environmental standard of the photochemical oxidant is 0.06 ppm or less as an average value measured per hour.
The ozone gas concentration measurement performed by the automatic measurement method described above measures ozone in the air by using a method of bubbling a gas to be measured in a neutral potassium iodide solution, and detecting the concentration by using a color developing reaction of generated iodine, or a method of detecting the concentration by using absorption of ozone in the ultraviolet region. Although these measurement methods can measure a slight amount of a gas, i.e., a few ppb of a gas, they have drawbacks that apparatuses are enlarged, complicated, and incapable of simple measurements. Also, these apparatuses are expensive and require constant setup in order to maintain the accuracy. In addition, automatic measurements performed by these apparatuses always require electric power and also require periodic calibration (maintenance). Therefore, enormous costs are required to maintain these apparatuses, and power supplies, temperature-controlled installation environments, and standard gases must be secured. That is, these apparatuses impose many limitations.
To accurately investigate the gas concentration distribution in an environment, evaluate the influence on a local environment, and evaluate the influence of exposure to a human body, an individual must monitor the environment by using a readily portable measurement method. For this purpose, the large-scale measurement apparatuses as described above cannot be used, and demands have arisen for the development of a an inexpensive, compact, and readily usable measurement device such as a gas sensor and a simple measurement method.
Recently, ozone is attracting attention because it has strong sterilizing power (oxidizing power), and changes into oxygen and produces no harmful substance after decomposition. This is extending the use of ozone to various industrial fields such as water processing, food sterilization, and paper bleaching. Accordingly, reference values of 100 ppb and 8 hrs are set for the ozone concentration as labor environmental standards. In a factory using ozone, it is of course necessary to install ozone alarms, and it is also necessary to manage the state in which each worker works within the range of the labor standards. This requires a measurement device that can be carried by a worker.
Under these circumstances, ozone gas measurement techniques have been presently extensively developed. Examples are a semiconductor gas sensor, solid-state electrolyte gas sensor, electrochemical gas sensor, and quartz oscillating gas sensor. However, these sensors have been developed to evaluate short-time responses, and only few sensors have been developed for monitoring requiring measurement data accumulation. When it is necessary to accumulate measurement data, therefore, the sensors must always be operated. Also, a sensing unit of, e.g., the semiconductor sensor must be held at a few hundred ° C., so a large amount of electric power is always necessary to constantly operate the sensor.
Furthermore, the above-mentioned sensors have a sensitivity of about sub-ppm, and hence cannot measure concentrations in real environments, e.g., cannot measure 10 ppb of ozone. Although some semiconductor sensors react to 10 ppb of ozone, the sensor output is nonlinear with respect to the concentration, and the output value largely changes from one sensor to another. This makes comparison difficult when using different sensors. Also, the influence of another gas cannot be neglected in many cases.
There is also a method using a detector tube type gas measurement device. Unfortunately, this method has also been developed to locally measure a very-short-time concentration in a measurement point. This makes it difficult to use the method to accumulate measurement data.
In addition to the above-mentioned ozone gas analyzing techniques, ozone detecting paper carrying (having) starch and potassium iodide (attached) has been proposed as a simple, high-sensitivity ozone analyzing technique (reference 1: Japanese Patent No. 3257622). However, this technique disclosed in patent reference 1 requires a pump for forcedly drawing a gas to be detected, a light source for measurement, and electric power for driving a detector including the pump and light source. Also, a special sheet-like carrier (substrate) is necessary, and must be replaced whenever measurement is performed. This makes cumulative measurement difficult. In addition, the measurement using the detecting paper described above detects all photochemical oxidants instead of ozone. Furthermore, this method has problems in accuracy and reproducibility because produced iodine gradually evaporates.
As another simple, high-sensitivity ozone gas analyzing method, a technique using ozone detecting paper carrying indigo carmine has been proposed (reference 2: Anna C. Franklin, et al., “Ozone Measurement in South Carolina Using Passive Sampler”, Journal of the Air & Waste Measurement Association, Vol. 54, pp. 1312-1320, 2004). However, this ozone detecting paper has no sufficient sensitivity and cannot well measure a storage amount of 100 ppb×8 hrs as the labor environmental standard. A technique by which a membrane filter is placed on the surface of an ozone detecting sheet carrying a blue indigo dye, and the sensitivity is controlled by adjusting the thickness of the membrane filter has also been proposed (reference 3: “Operating Instructions for Ozone Monitor”, Part#380010-10, http://www.kandmenvironmental.com/PDFs/ozone.pdf).
Furthermore, as a simple, advanced ozone detecting device, the present inventors have proposed an ozone detecting device using porous glass containing, in pores, a dye that changes light absorption in the visible region by reacting with ozone (reference 4: Japanese Patent Laid-Open No. 2004-144729). This technique can measure ozone gas at high accuracy without any large-scale apparatus. However, even this technique requires electric power to drive a light source and detector during measurement, and also requires an expensive carrier, i.e., porous glass.